I am not a conservation expert, but I've had similar problems.

Paper usually crumbles because of its acidity, and I don't think
there are any amateur "fixes" for it. I would lay out the pieces as
gently as possible on a flat surface, and photograph or scan them -
that at least preserves the information.

I would not use anything like tape to hold it together, because the
adhesive will age, and damage the document. I prefer to make notes
or labels on separate pieces of paper and place them together, rather
than write on the original, but if you must write on it, India ink or
pencil won't bleed into the paper the way ballpoint ink, or magic
marker, does.

And I would slip the original into an "acid-free" clear plastic page
protector for storage, and file it away >from heat, light, water, etc.

tom klein, Toronto

My mother handed me an envelope that she said contained some sort of
document >from her mgf. I opened the envelope and there was crumbling
paper inside. i was really afraid to handle it. It's my ggf's
citizenship document I think, and I could see a date, 1892.

The document had been folded many times and the paper was brown along
the folds and the document had separated wherever it had been folded
so it is now in several separate squares.

What should I do to piece this document together without completely
destroying it?

Varda Epstein
Usually in Efrat, Israel, but visiting in Pittsburgh